Circuit arrays are used as multi-band front-end modules for mobile telephones. They are connected to the antenna of a mobile telephone at the antenna input. When the antenna comes into contact with an electrically-charged user, this can result in electrostatic discharge, also known under the name “electrostatic discharge (ESD).” These electrostatic discharges can create voltage peaks that are capable of destroying the circuit array. Thus, it may be necessary to equip circuit arrays with a device that protects against ESD.
Printed specification WO 00/57515 discloses circuit arrays that are equipped with a protective device against ESD. The protective device comprises an electric high-pass filter in which a capacitor is connected in series and an inductor is connected in parallel to the antenna input path.
The ESD protective device helps reduce the ESD impulse entering the circuit array directly through the antenna. In addition to the impulse entering the circuit array directly through the antenna, an electrostatic discharge can also generate a high voltage in the circuit array through ground coupling. This can occur, for example, because the control input normally used in a circuit is arranged either on the high voltage (high) or on a low voltage (low). In this case, the high voltage (high) is defined, for example, by the fact that it lies 2.3 V above the ground voltage of the circuit array. Because, in the case of a mobile telephone, as with many other devices based on signal transmission via antennas, the signal input runs from the antenna to the system's ground, an electrostatic discharge can also directly affect the ground voltage of the circuit array in a circuit array of the type mentioned above. As a result of directly coupling a control line to ground through the “high” condition, the voltage impulse resulting from an electrostatic discharge can not only affect the path, via the antenna, but also the circuit array, via the control line.
A high-pass filter allows all frequency components of a signal, beginning with a certain cutoff frequency, to pass through it virtually unimpeded. In general, however, only a very narrow frequency range is relevant to the further processing of the signal received by the antenna in the mobile telephone. For example, frequencies of between 1 and 2 GHz are used in mobile telephones based on the GSM, PCN or PCS standard. All other frequencies received by the antenna tend to be interfering and, therefore, must be filtered out.